Molecular biology of the Philadelphia positive leukaemias

The Philadelphia (Ph) chromosome is a small chromosome 22, which results from a reciprocal translocation between the long arms of chromosome 9 and 22, designated t (9;22) (q34;q11). It was first described in association with chronic myeloid leukaemia (CML), where 90% of cases examined are Ph-positive. A similar cytogenetic abnormality has also been identified in the acute leukaemias but in a much lower percentage. The ubiquitous nature of the translocation in CML suggested that it was causally implicated in the pathogenesis of the disease. Recent work at the molecular level has corroborated this idea. As a consequence of the translocation, the Abelson protooncogene (ABL), located on chromosome 9 is moved to chromosome 22 where it is joined to a truncated gene, known as BCR. The result of this genomic reorganisation is a hybrid gene encoding a novel chimaeric protein product with enhanced protein tyrosine kinase activity. It is thought that it is this activity which is necessary for the generation of the leukaemic phenotype. The t(9;22) has provided a model to illustrate how cellular proto-oncogenes can be activated by chromosomal translocation and has stimulated interest in investigating other chromosomal translocations in human malignancies.